This is related to Midya et al., Ser. No. 09/477,985, entitled xe2x80x9cSwitching Circuit and Method Therefor,xe2x80x9d and Midya et al., Ser. No. 09/478,024, entitled xe2x80x9cCircuitry for Converting a Sampled Digital Signal to a Naturally Sampled Digital Signal and Method Therefor,xe2x80x9d both filed on even date herewith, and are incorporated herein by reference.
This is also related to U.S. patent application Ser. No. 09/307,453, filed May 7, 1999, and entitled xe2x80x9cMethod and Apparatus for Producing a Pulse Width Modulated Signalxe2x80x9d and is incorporated herein by reference and assigned to the current assignee hereof.
This invention relates generally to digital amplifiers, and more specifically to noise shaping of a PWM signal.
In power delivery system applications it is desirable to provide maximum power to the load, and avoid dissipation of power throughout the system. In applications such as digital audio amplifiers, often pulse width modulation (PWM) is used to transform a digital signal into its analog component. These systems are often referred to as digital PWM audio amplifiers, and are used to reproduce signals stored on a compact disc (CD) in a 16 bit format, or in a higher resolution format of 18 to 20 bits, where information is stored according to pulse code modulation (PCM).
The CD signal is a digital signal, represented by series of ones and zeroes. A PWM signal with quantized duty ratio is generated from the digital signal by periodic switching between a high and low output voltage level. This quantization operation introduces noise, and disturbances to the output signal. It is desirable to limit the switching frequency in order to reduce the total amount of switching necessary.
The analog signal generated by a PWM is typically distortion free. The PWM signal is described by its period, magnitude, and phase. Each cycle of a PWM signal has a corresponding duty ratio. During the digital processing, the PWM duty ratio is quantized, resulting in a reduced signal-to-noise ratio (SNR). The quantized PWM signal introduces audible distortion and raises the noise floor of the audible frequency band. One prior art solution involves noise shaping the PWM signal using classical noise shaping to manipulate the entire signal in hopes to reduce the noise floor and force the noise out of the audible range. One method treats the PWM signal as a PCM signal, however, PWM is a nonlinear process and results in out-of-band noise mapping into the baseband. When using two-sided PWM, the right and left halves are noise shaped individually, resulting in half the over-sampling ratio.
During noise shaping the phase information of the signal is typically lost. Most prior art solutions seek to compensate for the phase loss without actually preserving the phase information. One such solution involves complicated filters which then requires sharp filter again at the output. Another solution characterizes the input data using a lookup table approach of stored information. This latter approach introduces greater memory requirements and inaccuracies introduced by the prediction methods.
There is a need for an accurate method of noise shaping which does not introduce the added memory requirements of prior art methods, and which preserves the phase information throughout the process.